Email validation pattern Unclosed Character Class

Question

I am using an email validation pattern I found at How to validate an email and it works fine except it allows a + in the first part of the email and that isn't allowed in my specs. The original code is

public static final String EMAIL_PATTERN = "^[_A-Za-z0-9-\\+]+(\\.[_A-Za-z0-9-]+)*@"
        + "[A-Za-z0-9-]+(\\.[A-Za-z0-9]+)*(\\.[A-Za-z]{2,})$";

protected boolean isInvalidEmail(String email) {
    pattern = Pattern.compile(EMAIL_PATTERN);
    matcher = pattern.matcher(email);
    return !matcher.matches();
}

I thought I could just remove the + from "^[_A-Za-z0-9-\\\\+] but I get a Pattern Syntax Exception: Unclosed Character Class . Can someone tell me why removing the + uncloses the class? Thanks!

Answer 1

You have to remove the \\\\+ portion.

\\\\ escapes the \\ character. \\+ escapes the + regex operator. Thus \\\\+ breaks down to \\+ which means match the literal + character.

Note: The + regex operator means match one or more of the preceding element.

Answer 2

The reason that it gives you Unclosed Character Class is because only removing the + now escapes the closing square bracket so it is considered part of the pattern. Hence, the class does not have a matching closing square bracket. As Jonny Henly mentions the solution is to remove the \\\\+ to align with your spec, but this gives the answer as to why it is unclosed.